Several hundred people gathered Sunday in the south-central U.S. state of Oklahoma at the former site of a federal building to mark the 20th anniversary of the terrorist bombing there that killed 168 people.

The service began with 168 seconds of silence. Survivors and tearful relatives of the dead read the names of each victim.

Former President Bill Clinton was among the speakers at the Oklahoma City gathering.

"For 20 years you have honored the memories of your loved ones, you have inspired us with the power of your renewal, you have reminded us that we should all live by the Oklahoma standard; service, honor, kindness," Clinton said.

"In doing these things, you had to choose to affirm our common humanity in the face of mad, crazy people who think that differences are all that matter enough to dehumanize innocent bystanders they never met," he told those gathered.

Message to ceremony

President Barack Obama sent a message, saying the passage of time will never extinguish the pain.

"But if those murderers hoped to terrorize the American people that day, to break our spirits or shatter the bonds that unite us, then they completely and utterly failed," Obama said.

On April 19, 1995, anti-government militant Timothy McVeigh parked a truckload of explosives in front of the Alfred P. Murrah federal building and detonated it. The blast was strong enough to damage buildings for 16 blocks.

The attack killed 168 people, including 19 children in the building's day care center, and wounded nearly 700 others. At the time, it was the deadliest act of domestic terrorism in U.S. history.

McVeigh said the attack was to avenge a deadly government siege of a Texas compound belonging to a group of religious fanatics wanted on several federal charges.

McVeigh never showed any remorse for his crime and was executed in 2001. His accomplice, Terry Lynn Nichols, remains in prison for life.